Ducklings dead in shell during hatching

I usually hatch our Australian Spotted duck eggs in my Little Giant still air incubators with automatic egg turners. I use dry incubation because it is humid here and I hatch in a separate Little Giant incubator that is slightly cooler with increased humidity. I hatch both ducklings and chicks in the same incubator hatcher but I use forced air during incubation of our chicken eggs and still air for our duck eggs so they are not together during incubation. I have been hatching successfully for over a quarter of a century but I still don't know enough about hatching failures to figure out why I can hatch ducklings just fine with my cheap styrofoam incubators while someone with a nice cabinet incubator has not been able to hatch our eggs as well.

I gave someone at least 12 dozen eggs to hatch and she has only hatched about a dozen ducklings. In one hatch she had three breeds of ducks hatching together and only one of each breed hatched so it is not just our eggs that are not surviving hatching. At the same time she is having more successful hatches in the same environment. Our ducklings have reached full term in her incubator but then they don't make it out of the shell in her hatcher (section in her incubator) for some reason. The losses are actually astonishing but I don't know why so many fail to hatch.

I told her I would only charge her for live hatches so I am not even getting feed money by practically giving away eggs thinking they would hatch successfully for her. We both have wait lists of people wanting ducklings too. The financial loss is disappointing but the loss of life is a bigger concern for me.

I know that if there was a problem with the eggs they would have died during the early stages of development and these have been late term deaths. They are alive at lockdown and in a short time they are mostly all dead. What could be causing that?

I know she does not wash the eggs and we only use a facial brush to lightly rinse them in warm (skin temperature) water. We rinsed some of them, but not all of them, before they left here for a drive that is about an hour away. I don't think they would have been shaken too much from here to the incubator and she has not said there was any problem with the air cells. I don't know how many bacterial deaths she may have had but these were full term ducklings that should have hatched.

She gets eggs from a poultry auction as well as from individuals and she also buys birds from the auction while we have a closed flock and we only hatch our own eggs for the most part. I usually run our incubators year round and I clean the hatching incubator every few days with antibacterial dish soap but our incubation incubators rarely get scrubbed down unless we get an early hatch or a rotten egg leak. I don't know how she goes about cleaning her hatcher since it is indide her incubator with developing eggs. Again, I think if bacteria were the problem they would not make it full term.

What causes late deaths during hatching? Is there any possibility the eggs are at fault or is it the incubator/hatcher that is defective in some way? I don't know her temp or humidity during hatching but I suspect too high humidity could be either drowning the ducklings or reducing oxygen to the point of suffocation. I don't know if positioning is an issue but it would not be responsible for so many deaths.

I am not sending any more eggs to her for hatching since we have ducks on nests and my daughter is doing the incubation for me. I don't check my temps or humidity because I have on time hatches so there is no need to adjust anything. Because too high temps are more fatal than too low temps I did set up the incubators towards the low end of the range. I open the lids a couple of times a week to shuffle the eggs down through the stack of incubators until they get to the hatcher and I candle as I shuffle to remove any clear eggs or quitters. I also open my hatching incubator to remove dry babies and egg shells as well as to candle during hatching if it seems necessary. I have assisted hatching birds at times but I give them time to struggle on their own most of the time. I don't consider myself an expert but I am experienced when it comes to certain things that can go wrong during hatching.

I have no idea what has gone wrong so I can't offer her any advice. I have people asking me for advice all the time and it is our goal to help educate people to be successful so I really want to know what went wrong. I know it won't save all the ducklings that died but maybe it will save some in the future. I certainly have losses when I hatch but the only times I have lost an entire hatch was when I had temperature spikes during a heat spell (the ones that survived the high heat were bow legged) or when I added too much humidity during hatching. I have had ducklings pop right out in the dry incubator because they must have been started before they went into the incubator so I don't usually add very much humidity during hatching but at one time I read recommendations and tried following them, much to my disappointment. I have both a hands off and a hands on approach depending on the circumstances.

I tried to find information about what causes ducklings to die in the shell during hatching but without any specifics it is hard to know what it could be. I am hoping for suggestions while knowing that no one has the ability to know for sure with limited information.